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Measurement Concepts
2-19
Measurement considerations
There are a variety of factors to consider when making low level measurements. These con-
siderations are listed and summarized in Table 2-6. For comprehensive information on all mea-
surement considerations, refer to the Low Level Measurements handbook, which is available
from Keithley Instruments.
Table 2-6
Summary of measurement considerations
Considerations
Description
For V and
Ω
measurements:
See Section 3 for details
Loading effects
Circuit loading caused by a high impedance voltage source.
Cable leakage resistance
For unguarded measurements, leakage resistance in the triax cable
(between HI and LO) shunts the voltage to be measured.
Input capacitance (settling
time)
At very high resistance levels, effects of cable capacitance can slow
down measurement response time.
Guarding input cable
Eliminates the effects of leakage resistance for high impedance
measurements and input capacitance when using a long input cable.
For I measurements:
See Section 4 for details
Input bias current
Offset current of Model 6514 could affect low current
measurements.
Voltage burden
Offset voltage of Model 6514 could cause errors if it is high in
relation to the voltage of the measured circuit.
Noise
Noise generated by source resistance and source capacitance.
For Q measurements:
See Section 5 for details
Input bias current
Offset current of Model 6514 is integrated along with the input
signal, affecting the final reading.
External voltage source
Input current to Model 6514 should be limited to <1mA.
Zero check hop
Sudden change in the charge reading when zero check is turned off.
Auto-discharge hop
Sudden change in the charge reading when auto-discharge resets the
charge reading to zero.
Summary of Contents for 6514
Page 203: ...14 CommonCommands...
Page 207: ...15 SCPISignalOriented MeasurementCommands...
Page 235: ...18 PerformanceVerification...
Page 254: ...19 Calibration...
Page 274: ...20 RoutineMaintenance...
Page 278: ...A Specifications...
Page 282: ...B StatusandErrorMessages...
Page 288: ...C GeneralMeasurement Considerations...
Page 294: ...D DDCEmulationCommands...
Page 299: ...E ExamplePrograms...
Page 306: ...F IEEE 488BusOverview...
Page 320: ...G IEEE 488andSCPI ConformanceInformation...
Page 324: ...H CalibrationOptions...